Field of the Invention
This invention relates to a tone signal generating apparatus suitable for use in an electronic musical instrument.
There have been proposed various kinds of tone signal generating apparatuses in which tone signals are electronically produced. Among such tone signal generating apparatuses, there is known one in which a musical tone signal is formed by repeatedly and sequentially reading from a memory each of a plurality of groups of data representing preselected portions of the musical tone signal. In such conventional tone signal generating apparatus, when the tone signal S to be generated has an envelope L as shown in FIG. 1, sampling data (or waveform data) of portions A1, A2, . . . of the tone signal S are previously stored in a memory. In this case, time length of each of the portions A1, A2, . . . is set to one period of the tone signal S. And, during the time T1 shown in FIG. 1, the sample data of the portion A1 of the tone signal S are sequentially and repeatedly read from the memory to generate the tone signal S. In the same manner, during the time T2 the sample data of the portion A2 are sequentially and repeatedly read from the memory to generate the tone signal S, during the time T3 the sample data of the portion A3 are read from the memory, and so on.
The above-described musical tone generating apparatus however has such a deficiency that a complicated signal processing need be performed at each of the boundary portions between the time T1 and the time T2, between the time T2 and the time T3, and so on. More specifically, for example, if the tone signal based only on the sample data of the portion A3 begins to be generated immediately after the completion of the generation of the tone signal corresponding to the time T2, the waveform of the tone signal abruptly varies at the boundary between the time T2 and the time T3. As a result, the generated musical tone becomes somewhat odd. To solve this problem, an interpolation structure has been proposed in the U.S. patent application No. 690,771 filed on Jan. 9, 1985 under the title "Tone Signal Generation Device For An Electronic Musical Instrument" and now U.S. Pat. No. 4,633,749 invented by different inventors from but assigned to the same assignee with the present application. This U.S. Ser. No. 690,771 is not prior art of the present application but referred to here by way of explanation of the advantages of the present application, and in that application, the sample data precedingly read from the memory and the sample data currently read from the memory are multiplied respectively by data decreasing from "1" to "0" with the lapse of time and data increasing from "0" to "1" with the lapse of time, as shown in FIG. 2. And, the thus obtained multiplication results are added together to form the tone signal in which the abrupt variation or discontinuity of the waveform has been compensated. This arrangement is however disadvantageous in that the construction of the circuit necessary for compensating for the abrupt variation of the waveform is rather complicated.